The invention relates to lithography systems, and relates in particular to direct-write lithography such as scanning-electron-beam lithography or scanning-optical-beam lithography.
In many lithography systems, the close proximity of certain types of features may cause pattern deformation in the resulting image. For example, in a scanning-spot-lithography system, a focused spot (of either electrons or photons) is scanned relative to a substrate to write the desired pattern. The latent-image is recorded in a chemical layer known as resist. In the case of a positive-tone resist, the area that is exposed is removed upon development, leaving the written pattern as topography in resist. In the case of negative-tone resist, the exposed areas remain after development, whereas all other areas are removed.
Although the intensity profile of the focused spot falls rapidly with distance from the center, the distribution extends over a large distance as shown at 10 in FIG. 1A. In particular, FIG. 1A shows a cross-section through the center of a simulated focal spot of a 0.7 numerical-aperture zone plate operating at λ=400 nm. The radius is measured at the half-maximum point. The intensity 12 that is outside the desired size of the spot acts as background, contributing exposure dose to neighboring regions. This background noise may adversely affect intensity imaging as well as thermal (or threshold) imaging. The image of the final exposed pattern includes exposed spots from other regions in the pattern. The addition of exposure dose in the unintended regions of the pattern is therefore a cumulative process. The intensity of a single focused spot may be plotted in log-scale as shown at 14 in FIG. 1B. The tail end 16 of the intensity plot acts as background to the desired dose, and delivers exposure dose to the neighboring regions in the resist. When the spots are numerous and close together (i.e., with dense features in the pattern), the cumulative dose tends to distort the exposed pattern. This creates unwanted proximity effects.
As shown, for example in FIG. 2A, a binary pattern 20 that consists of an isolated line 22, a first group of three lines 24 and a second group of three tightly spaced lines 26 may be imaged. The desired linewidth is the same for all the sets of lines. FIG. 2B shows the simulated image of the exposed pattern 30 including a pattern 32 for the isolated line 22, an image 34 for the first group of lines 24 and an image 36 for the second group of lines 26. The cross-section through the intensity profile of the image 40 is shown in FIG. 2C where the profile 42 is associated with the isolated line 22, profile 44 is associated with the first group of lines 24 and profile 46 is associated with the second group of lines 26. The response of resist to an image is approximated by a thresholding function, i.e., areas of the resist that receive a total-exposure dose more than a certain resist-threshold 48 are completely removed after development. Assuming this model, the resist profile after development is shown in FIG. 2D where it may be seen that the width of the isolated line 52, W is smaller than each of the widths of the semi-dense lines 54, W1. As seen from the intensity profile, this is due to the fact that the close proximity of the lines causes exposure dose to spill over into the spaces between the lines, and hence increase the linewidths. This phenomenon is so severe in the case of lines of high density 26 that these lines are completely washed out after development and not resolved at all, as shown at 56. This shows that proximity effects not only modify the intended linewidths of features but also affect the resolution of the lithography system.
There is a need therefore, for a proximity-effect-correction technique that efficiently and effectively provides improved imaging with high contrast and high-resolution.